LeetCode-17. 电话号码的字母组合

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
Example:

Input: "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:

Although the above answer is in lexicographical order, your answer could be in any order you want.

回溯法

有时思维定式真的好可怕，总想着怎么让digits[0]与digits[1]去搭配，但如果是三个数字组合呢，思维要崩了。唉，其实可以用一个空字符串变量来累加每个数字所对应的字符啊。当长度与digits长度相同时，就产生一个组合了。

class Solution {
private:
    unordered_map<char,string> mp{
        {'0',""},
        {'1',""},
        {'2',"abc"},
        {'3',"def"},
        {'4',"ghi"},
        {'5',"jkl"},
        {'6',"mno"},
        {'7',"pqrs"},
        {'8',"tuv"},
        {'9',"wxyz"}};
public:
    vector<string> letterCombinations(string digits) {
        vector<string> s;
        if(digits == "") return s;
        dfs(s,"",digits,0);
        return s;
    }
    void dfs(vector<string>& res,string str,string digits,int k)
    {
        if(str.size() == digits.size())
        {
            res.push_back(str);
            return;
        }
        for(auto &r:mp[digits[k]])
        {
            str+=r;
            dfs(res,str,digits,k+1);
            str.pop_back(); //还原
        }
    }
};